Optical scanning apparatus for a multi-layer record carrier, including a focus control circuit

ABSTRACT

An optical beam ( 8 ) in a scanning device is focused on an information layer ( 3 ) of a multi-layer optical record carrier. A focus control circuit ( 22 ) keeps the focal point ( 9 ) of the beam on the information layer. When the focal point jumps to another information layer, the focus control circuit does not monitor the so-called central aperture signal in order not to depend on the presence of information in the information layers. During the jump the characteristics of the feedback loop of the focus control unit are modified to avoid instabilities in the movement of the focal point during the jump. When the focal point is captured on the designated information layer, the characteristics of the feedback loop are restored.

This application is a Continuation of Ser. No. 08/688,481 filed Jul. 30,1996, now U.S. Pat. No. 5,757,744.

The invention relates to an optical scanning apparatus for scanning anoptical record carrier having at least a first and second superjacentinformation layers, comprising

an optical system including means for focusing a radiation beam to afocal point,

a focus actuator for controlling the focusing means to shift the focalpoint in a direction perpendicular to the layers,

a focus error detector for deriving a focus error (FE) signal indicativeof the direction and distance between the focal point and the layer tobe scanned,

a focus control circuit for deriving a focus actuator drive signal inresponse to the FE signal and supplying said drive signal to the focusactuator to cause the actuator to maintain the focal point substantiallyon the layer to be scanned,

a first controller for generating a binary loop-control signal whichassumes a first logical value controlling a modification of thecharacteristics of the focus control circuit after receiving alayer-jump command for changing the scanning from the first to thesecond layer and a second logical value controlling the restoring of thecharacteristics after receiving a transition of a binary close signal.

An optical record carrier may comprise several information layers toincrease its storage capacity. These layers may be scanned by a singleoptical beam which is brought to a focal point by means of an objectivelens. A focus control circuit keeps the focal point on an informationlayer, the circuit controlling the position of the objective lens inresponse to a focus error signal, which represents the deviation of thefocal point from the information layer. The focus error signal isderived from the radiation reflected or transmitted from the recordcarrier and intercepted by a photo-detection system. The radiation fromthe record carrier is also used to generate a so-called central aperture(CA) signal, which is a measure for the total amount of radiationreflected by the record carrier. The CA signal may be used to form aninformation signal representing the information read from the recordcarrier.

When a scanning apparatus starts a scanning session, it must move theobjective lens towards the record carrier. During this movement, thefocal point crosses the so-called entrance plane of the record carrier,a low-reflection boundary surface of a substrate of the record carrierthrough which the information layers are scanned. In order to avoid thefocal point to lock on the entrance plane, the focal control circuitchecks the amplitude of the CA signal. When the amplitude due to theentrance plane is below a certain level, the circuit will not lock thefocal point on the entrance plane, and continue moving the objectivelens towards the record carrier. When the focal point approaches theinformation layer of the record carrier, the high reflection of thislayer will cause the CA signal to cross said level, and the focuscontrol circuit will lock the focal point on the information layer.

The European patent application nr. 0 020 199 discloses a scanningapparatus comprising a focus control circuit for changing the focalpoint between two information layers of an optical record carrier. Whenchanging to another information layer, the feedback loop of the focuscontrol circuit is opened, the actuator is driven in the appropriatedirection, and the level of the high-frequency content of the CA signalis monitored. When the amplitude of the high-frequency content crosses acertain level, the focal point is close to the information layer, andthe feedback loop is closed. The focus control circuit then takes carethat the focal point follows the information layer.

A disadvantage of the know scanning apparatus is, that the changing thefocal point between layers does not operate anymore if one or bothlayers have not (yet) been provided with information. Since in that casethe CA signal does not comprise the high-frequency content, said levelwill not be crossed and the feedback loop will not be closed.

It is an object of the invention to provide an optical scanningapparatus that can reliably focus on all information layers of anoptical record carrier, whether the layers contain any information ornot.

The scanning apparatus according to the invention is theretocharacterized in that the optical scanning apparatus comprises

a second controller for generating the close signal in response to theFE signal and comprising an inhibitor for inhibiting the transition ofthe close signal in an interval around a zero-crossing of the FE signalin between subsequent zero-crossings corresponding to the informationlayers. It has turned out that a layer jump can be realized reliably byonly monitoring the FE signal, which is not affected by the presence orabsence of information contained in the information layers. This is incontrast with the known focus control circuits, which use both the FEsignal and CA signal. When using only the FE signal, a specialprecaution must then be taken when moving the focal point from one layerto the other. If the feedback loop of the focus control circuit were tobe kept closed when changing to another information layer, the actuatorwould acquire too much speed around the zero-crossing of the FE signalsituated in between two information layers. As a consequence, theactuator might shoot over the designated information layer. Theapparatus according to the invention solves the problem by opening thefeedback loop or part of it in an interval around said zero-crossing,thereby avoiding the undue speed increase.

An important contribution to the speed increase comes from thedifferentiating branch in the feedback loop due to the slope of the FEsignal around said zero-crossing. The part of the feedback loop to beopened is therefore preferentially the differentiating branch. The othertwo branches of the feedback loop, i.e. the proportional and integratingbranch, can, to a certain extent, control the speed of the actuatorduring the jump in response to the motion of the record carrier.

In another embodiment of the apparatus according to the invention, allbranches of the feedback loop are opened during a jump. The motion ofthe actuator during the jump can then be controlled independently fromthe motion of the record carrier.

A preferred embodiment of the scanning apparatus according to theinvention is characterized in that the inhibitor comprises a comparatorfor detecting upward crossings of a preset level by the absolute valueof the FE signal, and the second controller generates the transition ofthe close signal after the 2N^(th) upward crossing after the layer-jumpcommand. When subsequent information layers have subsequent ordinalnumbers, then N is the absolute difference in ordinal number between thefirst and second information layer. The FE signal has generally theshape of the known S-curve, having a zero value when the focal point isfar removed from an information layer and when the focal point islocated on the information layer, and with a different sign on bothsides of the information layer. When the focal point is moved throughthe information layers, each layer will give rise to an S-curve. Thecloseness of the neighbouring S-curves causes the emergence of a clearzero-crossing of the FE signal in between two information layers. Thecomparator detects when the absolute value of the FE signal crosses thepreset level from a value below to a value above the preset below. Sucha crossing is called an upward crossing. The first upward level crossingafter start of the layer jump belongs to the S-curve of the informationlayer the focal point has just left. If the feedback loop were to beclosed on the first upward crossing, as is common in the known scanningapparatuses, the focus control unit might drive the focal point back tothe information layer it has just left. For a jump to a neighbouringlayer the inhibitor according to the invention will allow that thefeedback loop will only be closed after the 2^(nd) upward levelcrossing. The 2^(nd) upward level crossing occurs in the S-curve of theneighbouring information layer. When the loop is closed after thiscrossing, the focus control circuit will cause the focal point to movetowards the designated neighbouring information layer.

The closing of the loop may be delayed after the detection of the2N^(th) upward level crossing to provide a stable landing of the focalpoint on the designated information layer. The delay may be up to thefirst zero of the S-curve after the upward crossing, i.e. up to themoment the focal point passes the designated information layer. Thescanning apparatus according to the invention is thereto characterizedin that the FE signal has substantially a zero value when the focalpoint coincides with one of the layers, and the inhibitor comprises azero-crossing detector for detecting zero-crossings of the FE signal,and the second controller generates the transition of the close signalsubstantially at the detection of the first zero-crossing of the FEsignal occurring after the 2N upward crossings.

The delay may also be up to the first maximum of the S-curve after theupward zero-crossing, thereby allowing a longer deceleration before thefocal point is on the designated information layer. The scanningapparatus according to the invention is then characterized in that theinhibitor comprises a zero-crossing detector for detectingzero-crossings of the derivative of the FE signal, and the secondcontroller generates the transition of the close signal substantially atthe detection of the first zero-crossing of the derivative occurringafter the 2N upward crossings.

Instead of inhibiting the closure of the loop until the 2N^(th) upwardcrossing of the FE signal, the closure may also be inhibited by a timer.The timer must inhibit the closure at least to past the zero-crossing ofthe FE signal which precedes the zero-crossing appertaining to thedesignated layer. When the speed of the actuator during the layer jumpand the distance between layers and the number of layers to jump isknown, the duration of the inhibition can be calculated and thispredetermined time can be programmed in the timer. In this embodiment,the scanning apparatus according to the invention is characterized inthat the inhibitor comprises a timer for generating a binary timingsignal having a transition after a predetermined time which depends onthe absolute difference in ordinal number between the first and secondinformation layer, and the second controller generates the transition inthe close signal after the transition in the timing signal.

In the embodiment having a timer, a further delay after lapse of thepredetermined time may be introduced before the feedback loop is closed.The further delay may be up to the detection of the first zero-crossingof the FE signal after the lapse of the predetermined time. The furtherdelay may be also up to the detection of the first zero-crossing of thederivative of the FE signal.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

In the drawings

FIG. 1 shows schematically a scanning device according to the invention,

FIG. 2 shows the form of the focus error signal as a function of theposition of the focal point,

FIG. 3 shows schematically the focus control circuit,

FIG. 4 shows an embodiment of the first controller,

FIG. 5 shows an embodiment of the second controller,

FIG. 6 shows the form of the focus error signal and the central aperturesignal as a function of the position of the focal point,

FIG. 7 shows an adjustable differentiator,

FIG. 8 shows a second and third embodiment of the second controller,

FIG. 9 shows a fourth embodiment of the second controller and

FIG. 10 shows part of a fifth embodiment of the second controller.

Identical reference numerals in the different Figures denote identicalelements.

FIG. 1 shows an optical scanning apparatus according to the inventionfor scanning an optical record carrier 1. The figure shows across-section of the record carrier, which has three superjacentinformation layers 2, 3, 4 each spaced by a distance δ. Each of thelayers may be scanned separately through a transparent substrate 5,which has an entrance surface 6 forming a boundary between theenvironment and the record carrier. Information layers 2 and 3 arepartially reflective in order to make the scanning of information layer4 possible through layers 2 and 3. The information may be encoded in theinformation layers in the form of pits, bumps, areas having a reflectionor magnetization different from the surrounding area. The record carriermay be disc shaped, rotatable around its centre or of rectangular shape.

The scanning apparatus has an optical system for generating an opticalbeam used for scanning the record carrier. The optical system comprisesa radiation source 7, for instance a semiconductor laser, forming aradiation beam 8. The radiation beam is converged to a focal point by afocusing means 10, for instance a single- or multi-element objectivelens. The objective lens may be moved along its optical axis, the z-axisi.e. in the directions of arrow I1, by means of a focus actuator 12. Thefocus actuator may be a linear motor in the form of a magnet in amagnetic coil or a magnetic coil within one or more magnets. The motionof the objective lens can shift focal point 9 through the informationlayers. Radiation reflected by record carrier 1 is converged byobjective lens 10 via a beam separator 13 on a detection system 14. Thebeam separator may be a normal beam splitter, a polarizing beam splitteror a grating. Detection system 14 comprises in general several radiationsensitive detection elements. The electrical output signals of theelements are fed in a signal forming circuit 15. The circuit forms thesum of the electrical signals, which is output as the central aperture(CA) signal. The CA signal is a measure for the total amount ofradiation reflected by record carrier 1. The high frequency content ofthe CA signal represents information encoded in the information layers.

Signal forming circuit 15 forms a radial error signal from the outputsignals of the detection system, which represents the distance betweenthe centre of the focal point and the centre of a track to be followedon an information layer. A radial control system for keeping the focalpoint on the track is known from inter alia U.S. Pat. No. 5,321,676, andis not elaborated nor shown in the Figures.

Signal forming circuit 15 also forms such a combination of theelectrical output signals of detection system 14 as to obtain a focuserror (FE) signal. The value of the FE signal represents the directionand magnitude of the deviation of the focal point from the informationlayer being scanned. The way of combining the electrical signals, thelay-out of the detection elements and the form of beam separator 13depends on the method used for forming the FE signal. Possible methodsare the so-called astigmatic method, the Foucault method and thebeam-size method, known from inter alia U.S. Pat. No. 4,023 033,European patent application nr. 0 583 036 and U.S. Pat. No. 4,724,533respectively.

FIG. 2 shows the form of the FE signal as a function of the displacementof the focal point in the z-direction, i.e. in a direction along theoptical axis of objective lens 10, for two crossings of informationplanes. The FE signal is substantially zero at positions 16 and 17spaced by δ, where the focal point coincides with information layer 2and 3 respectively. The FE signal has a positive maximum 18, 20 and anegative maximum 19, 21 on both sides of each zero-crossing 16 and 17.

In FIG. 1 the FE signal is input to a focus control circuit 22, forderiving a focus actuator (FA) signal in response to the FE signal andsupplying said drive signal to focus actuator 12 to cause the actuatorto maintain focal point 9 substantially on the information layer to bescanned. FIG. 3 shows schematically the lay-out of focus control circuit22. The FE signal is input in a filter circuit 23, comprising across-over filter and, possibly, an amplifier. The output signal of thefilter circuit is used as input for three circuits. The first circuit isa linear amplifier 24 for obtaining a proportional focus control action.The second circuit is a differentiator 25 for obtaining adifferentiating focus control action. The third circuit is an integrator26 for obtaining an integrating focus control action. The output signalsof the three circuits are combined in an adder circuit 27 to produce theFA signal for focus actuator 12. Integrator 26 has an additional inputsignal from a ramp-increment circuit 28. This circuit can generate aconstant level signal, which, when input on the integrator causes the FCsignal level to increase in the form of a ramp. The inputs of linearamplifier 24 and differentiator 25 and the two inputs of the integrator26 can be interrupted by switches 29, 30, 31 and 32 respectively,arranged in the signal paths to the inputs. The switches are controlledby a loop controller 33 in dependence on a loop control (LC) signal. TheLC signal may comprise a series of signals transmitted in parallel. Theopening and closing of the switches modifies the characteristics offocus control circuit 22. The actual operation of the control will beexplained below. Integrator 26 is preferably switched on and off at itsone or more inputs instead of at its output. Switching at the outputentails removing or adding to the FA signal the Output of theintegrator,which may have a significant level, thereby disturbing thefocus control. The embodiment of control circuit 22 shown in FIG. 3controls the motion of the focus actuator during the layer jump by meansof the regularly increasing output level of integrator 26. In analternative embodiment the motion of the focus actuator is determined bya relatively short pulse supplied to adder circuit 27, after which theactuator moves without further powering. The pulse may, for example,have a duration of 1 ms, after which the actuator continues to move 2ms, giving a total time of about 3 ms to move the focal point from onelayer to another layer over a distance of 25 μm.

The combination of detection system 14, signal forming circuit 15, focuscontrol circuit 22, focus actuator 12 and focusing means 10 forms afeedback loop for keeping focal point 9 on an information layer to bescanned. Linear amplifier 24, differentiator 25 and integrator 26 formthree branches of the feedback loop.

The binary LC signal in FIG. 1 is generated by a first controller 34.FIG. 4 shows an embodiment of the controller. The LC signal assumes afirst logical value after receiving a layerjump (LJ) command forchanging the scanning from one information layer to another one. The LCsignal assumes a second logical value after receiving a transition in abinary close (CL) signal. The LC signal modifies the characteristics ofthe feedback loop via loop controller 33 the moment the command isreceived to change information layers. The signal restores thecharacteristics of the feedback loop when a transition in the CL signalis detected and the focal point is close to the information layer wherethe scanning is to be continued. A circuit 35, possibly a set-resetflip-flop, modifies the LJ signal and CL signal into the LC signal.

First controller 34 also comprises a jump control signal generator 36for supplying a jump (JU) signal to the focus control circuit insynchronism with the LC signal for making focal point 9 move in thedirection of the information layer to be scanned. On receipt of the LJcommand, the generator outputs the JU signal to focus control circuit22. This signal closes switch 32 of the focus control circuit, uponwhich ramp-increment circuit 28 sends a constant level signal ofpositive or negative polarity to integrator 26, causing the focusactuator to move over a distance and in a direction indicated by theconstant level signal and determined by the number of information layersto jump and the direction of the jump.

The CL signal used as input for first controller 34 is generated in asecond control circuit 37 in response to the FE signal and the CAsignal. The CA signal is used when the focal point is moved through theentrance face towards the information layers. When changing the focalpoint between information layers, the CA signal need not be usedanymore, and the FE signal will be used to generate the CL signal. FIG.5 shows an embodiment of the second controller. The CA signal or alow-pass filtered CA signal is input to a comparator 38, which gives abinary ‘1’ output signal when the CA signal exceeds a predetermined CApreset value CAp. The value of CAp is set such that it is higher thanthe CA signal due to entrance face 6 and lower than the CA signal due toan information layer. The FE signal is rectified in a rectifier 39. Acomparator 40 compares the output of rectifier 39 with a predeterminedFE preset value FEp. The value of FEp is set such that it is higher thanthe peak value of the noise in the FE signal and smaller than themaximum value of the S-curve. The outputs of comparator 38 and 40 arecombined in an AND circuit 41, which gives a binary ‘1’ output signalonly if both its inputs are ‘1’. According to the invention the outputof comparator 40 is also connected to the input of a counter 42, whichcounts the number of ‘0’ to ‘1’ transitions at its input after alayer-jump command has been given. Thus the counter counts the number ofupward crossings of the preset level FEp by the FE signal. A comparator43 compares the output of counter 42 with a preset number 2N, where N isthe absolute difference in ordinal number of the information layersbetween which the layer jump is made. When the number of transitionscounted by counter 42 equals the preset number, comparator 43 outputs abinary ‘1’ signal. A switch 44 connects either the output of AND circuit41 or the output of comparator 43 to the CL-signal conductor, dependingon whether focal point 9 is to be focused on information layer 2 aftercrossing entrance surface 6, or a focus jump is made between informationlayers. The CL-signal conductor connects the CL signal to firstcontroller 34. The combination of rectifier 39, comparator 40, counter42 and comparator 43 forms an inhibitor, which inhibits the occurrenceof a transition in the C1 signal as long as the required number ofupward crossings in the FE signal have not yet been detected.

The operation of the control circuits will now be described withreference to the circuit diagrams in FIGS. 1, 3, 4 and 5 and thediagrams in FIGS. 2 and 6.

At the start of a scanning session, a layer-jump (LJ) command is givento first controller 34, which causes jump control signal generator 36 tosend a jump pulse to focus control circuit 22. The subsequent closing ofswitch 32 causes the focus actuator (FA) signal to ramp up or down,depending oil the sign of the signal generated by ramp-increment circuit28. Circuit 35 will output an LC signal, opening switches 29, 30 and 31,thereby opening the feedback loop, of which focus control circuit 22forms a part. The values of the FE and CA signals during the ramp areshown in FIG. 6 as a function of the displacement z along the opticalaxis of the objective lens 10. Second controller 37 monitors thesevalues and acts when both signals cross the appropriate preset values.The FE signal around position 45 of the entrance surface is equallylarge as the FE signal around the positions 46, 47 and 48 of informationlayers 2, 3 and 4 respectively, because signal forming circuit 15normalizes the FE signal, making it independent of the reflectivity of asurface or layer. Since the FE signal of the entrance surface crossesthe FEp level, comparator 40 will output a ‘1’ signal. However, thenot-normalized CA signal remains below the CAp level because of the lowreflectivity of the entrance surface. Comparator 38 will not output a‘1’ signal, and, since switch 44 connect the CL signal to the output ofAND circuit 41, no transition in the CL signal is generated. Hence, thefeedback loop remains open, and the ramp continues. When the focal pointapproaches position 46, where it coincides with information layer 2, theCA signal crosses the CAp level and, somewhat later, the FE signalcrosses the FEp level at position 49. AND circuit 41 will then output a‘1’ signal. Because switch 44 connects the output of AND circuit 41 tothe CL conductor during the start phase of a scanning session, the CLsignal is transmitted to first controller 34, where it resets circuit35. The value of the LC signal drops to ‘0’, closing switches 29, 30 and31, thereby closing the feedback loop. At the same time switch 32 isopened by circuit 33, terminating the ramp. Focus control circuit 22will now control the position of focal point 9 such that it moves fromposition 49 to 46, where it stays locked. A more detailed description ofthe initial focusing step may be found in U.S. Pat. No. 5,189,293.

When during a scanning session the focal point must jump layers, forinstance from information layer 2 to information layer 3, i.e. fromposition 46 to position 47 in FIG. 6, the above described, known methodused for initial focusing cannot be used. If the known method were to beapplied, level crossings of both the CA and FE signals would bemonitored. Since the comparator checks the absolute value of the FEsignal, comparator 40 will detect an upward crossing at position 50. TheCA signals of the information layers overlap to such an extent, that theCA signal remains above the CAp preset level during layer jumps. Hence,comparator 38 will also output a ‘1’ signal, causing AND circuit 41 tooutput a ‘1’ signal. In the known method, switch 44 connects the outputof AND circuit 41 with the CL conductor, so that the signal CL is set to‘1’, causing the feedback loop to be closed. In case the actuator doesnot move sufficiently fast at position 50, the focus control will pullthe focal point back to position 46. In case the actuator moves fastenough, the focal point will continue to move in the direction ofposition 47. However, around the zero-crossing of the FE signal atposition 51, where the focal point is half way the distance betweenlayers 2 and 3, the actuator will acquire a high speed due to the effectof the positive slope of the FE signal on the differentiator of focuscontrol circuit 22. The high speed makes capture of the focal point atposition 47 uncertain.

The focus control according to the invention solves this problem by amore advanced monitoring of the rectified FE signal. When performing thelayer jump from layer 2 to 3, counter 42 counts the number of upwardcrossing of level FEp by the rectified FE signal. When this numberequals the preset value 2 for this jump, which occurs at position 52,comparator 43 issues a ‘1’ signal. Since switch 44 connects the outputof comparator 43 to the CL conductor when performing layer jumps, the C1signal will be set to ‘1’, which will close the feedback loop. Theactuator will then travel from position 52 to position 47 withoutincurring too much speed, thereby ensuring a proper capture of the focalpoint on information layer 3. The focus control according to theinvention has thereby passed the zero-crossing at position 51 whiletemporarily opening the feedback loop.

When the focal point must jump N layers, the preset value of comparator43 will be set to 2N. The second controller must then detect 2N upwardcrossings of the rectified FE signal before the feedback loop will beclosed. During this time the JU signal ensures that the actuatorcontinues moving towards the designated information layer at a desiredspeed. The JU signal may impart any speed profile on the actuatormovement, such as an initial acceleration, followed by a constantvelocity movement and terminated by a deceleration.

In another embodiment of the optical scanning device according to theinvention, the modification of the feedback loop around zero-crossing 51comprises a change in the differentiator of the feedback loop. FIG. 7shows the diagram of an adjustable differentiator 25′, which, in thisembodiment, replaces differentiator 25. The input signal of thedifferentiator is connected to a first input of an operational amplifier51 via a capacitor 54. The second input of the operational amplifier isconnected to earth. Two resistors 55 and 56 are connected to the firstinput. A switch 57 allows to connect either resistor 55 or resistor 56to the output. Different values of the resistor provide differentstrengths of the differentiating action. The switch is operated viacircuit 33. When the focus is locked on an information layer, the switchwill be in such a position that a maximum differentiating action isobtained. During a focus jump, the switch will be in such a positionthat a smaller differentiating action is obtained, thereby avoiding thatthe actuator acquires too much speed when crossing positions like 51.During initial focusing, the feedback loop may opened by switches 29,30, 31 and 32 as described above.

The embodiment of the second controller as shown in FIG. 5 generates the‘1’ CL signal at the upward crossing of the FE signal at position 52. Ifa short braking distance between closing of the loop and capture of thefocal point on the designated information layer suffices, the ‘1’ CLsignal may also be given at position 47, i.e. at the first zero-crossingof the FE signal after the 2N^(th) upward crossing. A second embodimentof the second controller having this feature is shown in FIG. 8. Theelements used for the initial focusing are identical for the first andsecond embodiment of the second controller. The FE signal is fed in azero detector 58, which outputs a logical ‘1’ value during apredetermined time after a zero-crossing of its input signal has beendetected. The output signal of zero detector 58 is connected to an inputof an AND circuit 59, which has the output signal of comparator 43 atits other input. The output of AND circuit 59 is connected to the CLconductor via switch 44 when the scanning device makes layer jumps. A‘1’ value at the output of zero detector 58 only result in a ‘1’ valueof the CL signal if the output of comparator 43 is also ‘1’. In otherwords, a transition to ‘1’ of the CL signal will only occur at the firstzero-crossing of the FE signal after the 2N^(th) upward crossing of theFE signal. The inhibitor is formed by elements 39, 40, 42, 43, 58 and59.

In a third embodiment of the second controller zero detector 58 ispreceded by a differentiator 60, shown by dashed lines in FIG. 8. Thecombination of circuits 58 and 60 detects zero-crossings in thederivative of the FE signal. The detection of such a zero after the2N^(th) upward crossing results in a transition to a ‘1’ value of the CLsignal. In FIG. 6 this means that, when the upward crossing at position52 has been detected, the CL signal will go to ‘1’ at position 60, i.e.at the maximum value of the FE signal.

FIG. 9 shows a fourth embodiment of the second controller. The LJcommand is input to a timer 61, which outputs a binary timing signal. Inthis embodiment the inhibitor comprises only the timer. When the Ucommand indicates that a layer jump must be performed, the timer issuesa ‘0’ timing signal. After a predetermined time the timing signal makesa transition to the ‘1’ value. The duration of the predetermined time isset by the number of information layers the focal point has to jump, thedistance between the layers and the speed imposed on actuator 12. At theend of the predetermined time the feedback loop closes. The focal pointmust then be close enough to the designated information layer to captureproperly. In this embodiment the timer forms the inhibitor which assuresthat the feedback loop is not closed in a region around a zero crossingof the FE signal which does not pertain to an information layer, such asfor example zero crossing 51 in FIG. 6.

FIG. 10 shows a part of a fifth embodiment of the second controller, inparticular the inhibitor. The elements used for the initial focusing areidentical for the first and fourth embodiment of the second controller.The FE signal is input at a zero detector 62, which outputs a ‘1’ signalafter detecting a zero crossing of the FE signal. This output iscombined with the timing signal from timer 61 in an AND circuit 63, theoutput of which is connected to switch 44. A transition of the CL signalto ‘1’ occurs the moment zero detector 62 detects the firstzero-crossing of the FE signal after the predetermined time of timer 61has lapsed. At the end of the predetermined time the focal point must belocated before the zero-crossing of the designated information layer, sothat the subsequently detected zero-crossing causes the closing of thefeedback loop. In stead of zero-detector 62, a comparator likecomparator 40 may be used in order to close the feedback loop at apredetermined value of the FE signal unequal to zero. The fifthembodiment of the second controller makes the scanning device lesssensitive to inaccuracies in the travel of the actuator during thepredetermined time, because the loop always closes at a well definedposition, irrespective of the position of the focal point at the end ofthe predetermined time.

A sixth embodiment of the second controller comprises a differentiator64, shown by dashed lines in FIG. 10, before zero detector 62. Analogousto the third embodiment of the second controller, the transition of theCL signal to ‘1’ occurs when zero detector 62 detects the first zerocrossing of the derivative of the FE signal after lapse of thepredetermined time. The sixth embodiment provides more braking distancethan the fifth embodiment.

An even longer braking distance may be obtained, when the ‘1’ transitionof the CL signal occurs after the first upward zero-crossing of the FEsignal after lapse of the predetermined time. For the implementation acombination of the timing signal and the output of comparator 40 in FIG.5 is required. The first and second controller may be integrated in asingle electronic circuit. When the CA and FE signals are processeddigitally, the two controllers and the focus control circuit may beadvantageously combined in a single electronic circuit.

Although the disclosure of the invention has been made with reference toa scanning device for scanning optical record carriers, a person skilledin the art will readily understand that the invention is not limited tosuch a device, but encompasses scanning devices for any kind of objecthaving superjacent layers comprising some kind of information. Anexample is a scanning device for investigating superjacent layers of anintegrated semiconductor circuit by radiation having a wavelength forwhich the layers are at least partly transparent. The device scans thedifferent layers forming the circuit, while the information read fromthe layers is the structure of these layers.

What is claimed is:
 1. A focus control system for an optical scanningapparatus for scanning an optical record carrier having multiplesuperjacent information layers, the optical system comprising: a focuserror detector which derives a focus error signal indicating thedirection and distance between a focal point and one of the informationlayers being scanned; a focus control circuit which derives a focusactuator drive signal in response to the focus error signal and suppliesthe drive signal to a focus actuator to maintain the focus pointsubstantially on the information layer being scanned; first controllermeans for generating a binary loop control signal which assumes a firstlogical value controlling a modification of the functionality of thefocus control circuit after receiving a layer-jump command signal forchanging the scanning from a first to a second information layer and asecond logical value controlling the restoring of the functionality inresponse to a loop close signal; and a second controller means forgenerating the loop close signal in response to a prescribedcharacteristic of the focus error signal such that the generation of theloop error signal is inhibited in an interval around a zero-crossing ofthe focus error signal when the focal point is at a position in betweensubsequent information layers.
 2. The focus control system of claim 1,in which the modification of the functionality includes changing from aclosed loop mode of operation to an open loop mode of operation of thefocus control circuit.
 3. The focus control system of claim 1, in whichthe modification of the functionality includes altering an amplificationfactor of the focus control circuit.
 4. The focus control system ofclaim 1, in which the modification of the functionality includesinterrupting operation of a proportional branch of a PID controller ofthe focus control circuit.
 5. The focus control system of claim 1, inwhich the modification of the functionality includes interruptingoperation of a differentiating branch of a PID controller of the focuscontrol circuit.
 6. The focus control system of claim 1, in which themodification of the functionality includes interrupting an input signalat an input of an integrated branch of a pid controller of the focuscontrol circuit.
 7. The focus control system of claim 6, in which afurther modification of the functionality includes a pulse generatorsupplying a pulse to an input of the integrating branch.
 8. The focuscontrol system of claim 1, in which the prescribed characteristic of thefocus error signal is an equivalent of the 2N^(th) upward crossing of apreset level by the absolute value of the focus error signal, where Nrepresents one plus the number of information layers between the firstinformation layer and the second information layer.
 9. The focus controlsystem of claim 1, in which the prescribed characteristic of the focuserror signal is a first zero-crossing of the focus error signaloccurring after a 2N^(th) upward crossing of a preset level by theabsolute value of the focus error signal, where N represents one plusthe number of information layers between the first information layer andthe second information layer.
 10. The focus control system of claim 1,in which the prescribed characteristic of the focus error signal is afirst zero-crossing of a derivative of the focus error signal occurringafter a 2N^(th) upward crossing of a preset level by the absolute valueof the focus error signal, where N represents one plus the number ofinformation layers between the first information layer and the secondinformation layer.
 11. The focus control system of claim 1, in which thesystem further comprises a timer for generating a timing signal for atime period which depends on a number N, and the second controllerinhibits the generation of the loop close signal during the time period, where N represents one plus the number of information layers betweenthe first information layer and the second information layer.
 12. Thefocus control system of claim 1, in which the prescribed characteristicof the focus error signal is a first zero-crossing of the focus errorsignal after the time period.
 13. The focus control system of claim 1,in which the prescribed characteristic of the focus error signal is afirst zero-crossing of a derivative of the focus error signal after thetime period.
 14. The focus control system of claim 1, in which the loopclose signal is generated substantially at or after the occurrence ofthe prescribed characteristic of the focus error signal.
 15. An opticalscanning apparatus for scanning an optical record carrier with aplurality of superjacent information layers, comprising: an opticalsystem of focusing a radiation beam on a focal point; a focus actuatorfor controlling the optical system to selectively shift the focal pointin a direction perpendicular to the information layers; a focus errordetector which derives a focus error signal indicating the direction anddistance between the focal point and one of the information layers beingscanned; a focus control circuit which derives a focus actuator drivesignal in response to the focus error signal and supplies the drivesignal to the focus actuator to maintain the focus point substantiallyon the information layer being scanned; first controller means forgenerating a binary loop control signal which assumes a first logicalvalue controlling a modification of the functionality of the focuscontrol circuit after receiving a layer-jump command signal for changingthe scanning from a first to a second information layer and a secondlogical value controlling the restoring of the functionality in responseto a loop close signal; and a second controller means for generating theloop close signal in response to a prescribed characteristic of thefocus error signal such that the generation of the loop error signal isinhibited in an interval around a zero-crossing of the focus errorsignal when the focal point is at a position in between subsequentinformation layers.
 16. A method for scanning an optical record carrierhaving multiple superjacent information layers, comprising the steps of:converging a radiation beam to a focal point; deriving a focal errorsignal from radiation coming from the record carrier and indicating thedirection and distance between the focal point and a first layer beingscanned; using the focus error signal as an input to a feedback loop forcontrolling the position of the focal point to keep it substantially onthe first layer being scanned; changing the position of the focal pointfrom the first layer to a second layer to be scanned; controlling theposition of the focal point to keep it substantially on the second layerbeing scanned; and in which changing the position of the focal pointincludes: energizing a focus actuator; monitoring the focus error signalduring the change in position; modifying the characteristics of afeedback loop before a zero-crossing of the focus error signal betweenthe subsequent zero-crossings corresponding to the layers; and restoringthe characteristics of the feedback loop after the zero-crossing inresponse to the focus error signal.